This application is based on application No. JP 11-354948 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an improved solid scanning optical writing device, as well as a light amount correction method and light amount measuring device therefor, which are used to write images (latent images) on a photoreceptor using a PLZT light shutter array or an LED array.
2. Description of the Related Art
Various optical writing devices that turn ON/OFF light for each pixel using a PLZI light shutter array or LED array have conventionally been proposed as a device to form images (latent images) on photosensitive paper using a silver halide photosensitive material, film or electrophotographic photoreceptor. For example, as shown in FIG. 12(A), in the case of an optical writing head comprising many light shutter elements 31a and 31b aligned in alternating fashion in two rows in the main scanning direction, an image for one line is formed in the following manner.
First, the light shutter elements 31a of one row are controlled to turn ON/OFF based on the image data, and consequently, the light from the light source is turned ON/OFF. As a result, the surface of the photosensitive paper, etc. being conveyed is exposed and an image (latent image) is formed.
When the area of the photosensitive paper on which the image has been formed reaches the exposure position for the light shutter elements 31b of the other row, the light shutter elements 31b of the other row are controlled to turn ON/OFF based on the image data, whereupon the light from the light source is turned ON/OFF and an image is formed on the surface of the photosensitive paper. Through this operation, the intermittent image formed by the turning ON/OFF of the light shutter elements 31a and the intermittent image formed by the turning ON/OFF of the light shutter elements 31b are combined, and an image for one line is formed.
Incidentally, PLZT light shutter arrays and LED arrays cause line noise in the output image due to the variations in the amount of light passing through each optical element or the amount of light emitted. In order to eliminate this noise, the variations in the light amount are corrected by performing correction calculation based on information obtained through measurement of either the darkness of the output image or the amount of light from the light shutter array (shading).
Because it is relatively easy to reduce the cost and make the measuring device small in size, a method that directly measures the light amount from the light shutter array is preferred. However, it is not easy to accurately measure the light amount for each pixel when the pixel density is 400dpi, and to match the information to the image.
For example, in the case of the optical writing head shown in FIG. 12(A), if the light amount is measured with all of the light shutter elements 31a and 31b of double-rows ON, i.e., illuminated (double-row illumination) in order to measure the light amount per pixel when the light shutter elements 31a and 31b of double-rows are ON, a light amount distribution waveform shown in FIG. 12(B) is obtained. However, in order to identify the position (address) of each light shutter element 31a and 31b in such a light amount distribution waveform, an expensive device such as a linear scale in which absolute addresses are defined must be used. Alternatively, where an expensive device such as a linear scale is not used, the position of each light shutter element 31a and 31b is identified by measuring the light amount for each light shutter element when only either row is illuminated (single-row illumination) and obtaining a light amount distribution waveform shown in FIG. 12(C). The light amount for each pixel when the light shutter elements 31a and 31b of double-rows are illuminated is calculated by means of such processing methods as adding the light amount values measured per row. However, since this state in which only a single-row is illuminated is different from the original state in which double-rows are illuminated, strictly speaking, errors will occur in the light amount value between when the light amount measurement is performed and when actual exposure is carried out, resulting in image unevenness.
When a light shutter array, etc. is used for an optical writing head in actuality, it is often used in combination with image forming lenses (selfoc lens arrays) due to such issues as efficiency in the use of light and the need to ensure the distance to the exposure surface. FIG. 12(A) shows the positional relationship between a selfoc lens array and the light shutter elements 31a and 31b of a PLZT light shutter array. The selfoc lens array comprises multiple rod lenses 35a that are combined such that one lens is placed between two lenses lengthwise, and the output light from each light shutter element 31a and 31b is formed into an image on the exposure surface via their corresponding rod lenses 35a. 
In the construction described above, the factors that hinder the uniformity of exposure when all of the light shutter elements are illuminated include (i) errors in the alignment of the light shutter elements formed on the PLZT light shutter chips (processing errors), (ii) positioning errors when the multiple light shutter chips are mounted on a substrate comprising glass or other materials, (iii) optical performance errors for each rod lens 35a of the selfoc lens array (depth of focus, chromatic aberration, rod lens alignment accuracy, etc.), and (iv) errors in the geometrical positioning of the light shutter elements 31 and the rod lenses 35a (assembly errors). Among these types of errors, errors in the optical performance of each rod lens 35a are the most significant, and often comprise the main reason for variations in light amount.
These errors cause a phase difference between the light amount distribution waveform obtained when only the light shutter elements 31a of one row are illuminated (curve L1) and the light amount distribution waveform obtained when only the light shutter elements 31b of the other row are illuminated (curve L2). In the presence of such a phase difference, the amplitude of the light amount distribution waveform when the light shutter elements 31a and 31b of double-rows are illuminated (curve L3) increases, resulting in so-called oscillation. For comparison purposes, FIG. 13(B) shows the light amount distribution waveforms in each case under ideal conditions in which there is no phase difference. In other words, even if the transmitted light amounts for each light shutter element 31a and 31b are adjusted such that they are all the same when the rows are separately illuminated, the oscillation itself that occurs when double-rows are illuminated cannot be reduced.
The present invention was created in view of the situation described above. An object of the present invention is to provide an improved solid scanning optical writing device and light amount measuring device and driving method therefor. Another object of the present invention is to provide (i) a solid scanning optical writing device having an improved light amount adjustment function, and (ii) an improved light amount adjustment method in the above device, and more particularly, to provide a solid scanning optical writing device, light amount correction method and light amount measuring device therefor that can essentially accurately measure the output light amount for each optical element and perform high-quality light amount correction based on the measurement data.
In order to attain these and other objects, the solid scanning optical writing device according to one aspect of the present invention comprises a solid scanning optical writing device that controls the many optical elements aligned in an alternating fashion in two rows in the main scanning direction to turn them ON/OFF based on the image data, wherein said device has a light amount measuring unit that includes a light amount sensor to measure the output light amount transmitted by each optical element, and that measures the output light amount and the position of each optical element by alternately performing single-row illumination of the optical elements and double-row illumination of the optical elements on a time sharing basis while the light amount measuring unit is moved in the main scanning direction.
The light amount measuring device according to one aspect of the present invention comprises a light amount measuring unit including a light amount sensor to measure the output light amount for each of the many optical elements aligned in an alternating fashion in two rows, a means to move the light amount measuring unit forward and backward in the direction of alignment of the optical elements, and a means to adjust the position of the light amount measuring unit, wherein single-row illumination of the optical elements and double-row illumination of the optical elements are alternately performed on a time sharing basis while the light amount measuring unit is moved in the main scanning direction, and the output light amount and the position of each optical element are measured.
The light amount correction method for the solid scanning optical writing device according to one aspect of the present invention is a light amount correction method for the solid scanning optical writing device that controls many optical elements aligned in an alternating fashion in two rows in the main scanning direction to turn them ON/OFF based on the image data, said method comprising (i) a light amount measuring process in which single-row illumination of the optical elements and double-row illumination of the optical elements are performed alternately on a time sharing basis, and the position and output light amount for each optical element are measured while the light amount measuring unit is moved in the main scanning direction, and (ii) a light amount correction process in which the correction amount for the output light amount for each optical element is calculated based on the data regarding the optical element output amount data obtained in the previous light amount measuring process. More specifically, the output signals from the light amount sensor of the light amount measuring unit are sampled in synchronization with the timing for the time-shared driving, and from among the data obtained by means of the sampling, the position of a prescribed optical element is specified from the amplitude of the sampling data when all of the optical elements of a single-row were illuminated, and the data from one sample before or after the sample by which the position of the optical element was specified is deemed the output light amount for the optical element when all of the optical elements of both rows are illuminated.
Using the method described above, the position of a prescribed optical element may be specified from the amplitude of the light amount data during single-row illumination, and the output light amount for the optical element during double-row illumination, the position of which has been specified, may be obtained from the light amount data during double-row illumination. As a result, the output light amount for each optical element may be essentially accurately measured, and high-quality shading correction (correction for variations in light amount) may be performed.
In addition, the light amount correction method for the solid scanning optical writing device according to another aspect of the present invention measures the position and output light amount for each optical element using the light amount correction method having the characteristics described above, detects the difference in output light amount between adjacent optical elements from the output light amount data for each optical element during double-row illumination, and calculates the correction amount for the output light amount for each optical element based on this difference in output light amount. In this case, it is preferred that the correction amount for the output light amount for each optical element based on the difference in output light amount be calculated by adding at least one of the following parameters: (i) the visual characteristic of the output image in a dark image area, (ii) the visual characteristic of the output image in a halftone image area, and (iii) the visual characteristic of the output photosensitive material.
Using the method described above, the oscillation areas in the light amount distribution waveform during double-row illumination due to the phase difference between the two rows of optical elements caused by the optical performance errors of the selfoc lens array may be detected through repeated light amount measurement, and the output light amount for each optical element is re-corrected by adding such parameters as the visual characteristic of the output image in terms of darkness. Consequently, even if the oscillation areas and the other areas (uniform areas) are adjacent to each other or coexist in a mixed fashion, uniform and even images may be obtained.